In resistive ribbon printing, an electrically conducting ribbon, termed a donor element, is contacted by a print head containing a row of small closely spaced electrodes. These electrodes are individually driven by respective electronic circuits to inject currents locally into the ribbon. The underside of the ribbon is coated with a suitable ink which, when the ribbon is locally heated by the currents applied to it by the print head, transfers small dots (pixels) of ink onto a receiver element such as a piece of paper. The pitch of the pixels of ink thus printed is determined by the spacings of the electrodes in the print head. The density of a printed pixel is controlled by the amount of ink transferred from the donor element to the receiver element. The amount of ink so transferred as a printed pixel is a function of the current (and the heating effect thereof) applied to the donor element by a respective electrode of the print head. Electrically conductive ink donor elements are commercially available.
The pitch of the ink pixels i printed on a receiver element is an important factor in determining the visual quality of an image thus printed. The finer the pitch of the pixels, the finer is the quality of the printed image. It is desirable for this reason to be able to print with a fine pitch, for example, greater than 300 dots per inch (DPI). However, pitches this fine or finer become increasingly more difficult to obtain because, among other things, of the very Close spacings required of the electrodes in the print head. For example, a 1000 DPI system requires that the head electrodes be spaced on one mil (25 micron) centers, and since the electrodes must also be insulated from each other, the actual size of each electrode has to be of the order of 16 microns square. It is difficult to manufacture, with the quality and precision required, electrodes that are this small. Moreover, since the electrodes must rub with a slight force against the dye donor element in order to make good electrical contact to it during printing of an image, the electrodes are gradually worn away. The rates at which the electrodes are worn are determined by such factors as the material of which they are made, the amount of pressure of the electrodes against the donor element, and the wear characteristics and sizes of the electrodes.
It is desirable to provide a print head for a thermal printer, such as described above, in which the individual electrodes of the head are ultra-small and which can be made with great precision at relatively low cost. It is further desirable that the electrodes be highly wear resistant and that they provide reliable electrical contact to a donor element during extended operation of the printer.